Inorganic papers and methods of making same



United. States Patent 3,005,745 INORGANIC PAPERS AND METHODS OF MAKING SAME Robert J.-H0lmes, Huntington, N.Y., assignor to General Aniline & Film Corporation, New York, N.Y., a corporation of Delaware No Drawing. Filed Mar. 4, 19,57, Ser. No. 643,500

16 Claims. (Cl. 1 62152) This invention relates to new, useful and improved processes for the preparation of paper and paper-like products particularly in sheet form, from minerahsubstances and the products resulting therefrom.

Papers and the like prepared from mineral or inorganic substances have been developed as specialty papers for use in numerous fields of application Where the common cellulosic base type paper was found to be sadly deficient in many desired properties. In its electronics applications, paper finds extensive use in capacitors as a dielectric material. However, the dielectric properties of paper are greatly inferior to those of mica'and the various ceramics used for such purposes. In addition, at elevated temperatures the cellulosic papers are practically worthless due to the degeneration of the organic cel-lulosic mat- 'ter. On'the other hand, dielectric materials prepared from the inorganic base substances such as glass, ceramics, mica and the like do not sufier from such deficiencies as described above for the cel-lulosic papers. However, in contradistinction to the advantages of such inorganic materials for the above enumerated properties, these inorganic dielectric substances have certain inherent properties Which normally render them undesirable in many instances. Thus the inorganic dielectrics have not found extensive or, indeed, practical application in the electronics field and other allied fields where flexibility and strength are necessary and/or desirable characteristics of the dielectric product, and this is especially 'so Where the product must be of paper thinness, that is, normally less than about 0.01 inch in thickness. Heretofore attempts to prepare such products employing organic binders and in particular resins such as are used in conventional paper making processes did not produce a satisfactory product. First, itrwas found that in. view of the-.difliculty in coatingwet inorganic fibers, excessive amounts of such resinsare required. This difficultyin coating the mineral fibers manifested itself in the formation of resin clots rather than in a desired and necessary uniform distribution of; the resin. Secondly, finished sheetsprepared -from such earlier procedures did not have the qualities of the fibers and did not therefore resemble a paper product but rather resulted in a plastic product reinforced'with an inorganic filler, and-this because of the high ratio of resin to fibers required. Other attempts toovercome these difliculties whereon the one hand the fibers are precoated and on the other hand organic solvents are employed in lieu of Water as the liquid dispersing medium for the fibers did not prove successful. The products. resulting from these manipulations resembled loosely carded webs. and were not possessed of the characteristics of a-paper sheet. Still another problem arising from the attempts to use the conventional resins developed for use with cellulose pulps is that many of such resins were developed for use at pH values of from about 6 to 8 and these resins were found to be completely inoperative at the pH values (pH about 3 to 4) normally employed in the making of inorganic papers. v

It is an object of this inventionto provide a process for the preparation of paper and/ or paperlike products from inorganic materials.

It is a further object of this invention to provide a process whereby sheet-like materials may be prepared from mineral fibers and the like.

It is a still further object of this invention to provide strong and flexible sheet-like materials derived from inorganic base substances. 7

:It is another object of this invention to provide processes for the manufacture of inorganic papers employing a polyvinyl lactam-containing polymeric material.

It is another object of this invention to provide inorganic papers containing a polyvinyl lactam-containing polymeric material.

Other objects will appear hereinafter as the description proceeds.

It was unexpectedly discovered that in the preparation of paper and other sheet-like materials having as a major basis therefor inorganic fibers or flakes, the incorporation during the paper-making process of minor amounts of a polyvinyl lactam-co'ntaining polymeric materials in the paper-making composition produced a product having all of the advantages ascribable to the inorganic constituents of the paper but still nevertheless possessed of the important characteristics. of strength and flexibility not heretofore obtainable in such types of paper.

The general procedure for manufacturing the papers of this invention involves techniques similar to those heretofore employed in the paper-making art with the outstanding differences being the addition to the inorganic paper-making stock at some period in the paper-making process, between beating and deposition on the wire, of a polyvinyl lactam-containing polymeric material while maintaining the stock at a pH of from 1.5 to 2.5. More particularly, the general process involves dispersing the inorganic material in a relatively large volume of Water and agitating the mixture vigorously. The temperature may be any suitable temperature and preferably 65 to 95 F. and to the suspension is then added the polyvinyl lactam-containing polymeric material, and after additional mixing, it is necessary to insolubilize the polyvinyl lactam-containing polymeric material on to the mineral base. The pH is then adjusted to. between 1.5 to 2.5 and then the stock is again additionally agitated mildly and the resultant slurry processed in the normal manner to form a paper-like sheet.

The polyvinyl lactam-containing polymeric materials which may be used in the present invention include all of the polyvinyl lactam-containing water-soluble polymers,

copolymers and mixtures containing a minimum of 20% N-vinyl lactam. v

. Polymeric N'-vinyl lactams, as exemplified by polyvinylpyrrolidone (PVP, poly-l-vinyl-Z-pyrrolidone, poly-N'- vinyl-2-pyrrolidone, poly-N-vinyl-a-pyrrolidone) are by this time well known as extremely versatile chemicals which have found many uses in a variety of fields. They are White powders or colorless, horn-like or glass-like materials with a high softening point and have the highly desirable property of being soluble in Water. They are characterized by the following general structural formula:

wherein R represents an alkylene-bridge group necessary tocomplete 5, 6 or 7-membered heterocyclic ring systems, R represents hydrogen or a lower alkyl group such as methyl or ethyl, and n represents a number indicative of the extent of polymerization and is usually at least 3 or 4. They are obtained by polymerizing organic 5, 6

' or 7-membered ring compounds containing in their rings the NH-CO group such as, for example, N-vinyl-2- Patented Oct. 24., 1961,

3 pyrrolidone, N-vinyl--, -4, and -3-methyl-2-pyrrolidones, N vinyl 2,2-dimethyl-2-pyrrolidone, N-vinyl-Z-pyrrolidone, N-vinyl-e-caprolactam, N-vinyl-hexa-hydrophthalamidine and the like. Depending upon the extent of polymerization, they may have molecular weights ranging from at least about 300 up to 2,000,000 or more, and preferably from about 400 to about 70,000. Viscosity measurements are commonly used as an indication of the average molecular weight of polymeric compositions, the instant polymers being chracterized by a chain of carbon atoms to which the lactam rings are attached through their nitrogen atoms, as follows:

\ 3HOH2( JHCHz( LH-CH2 The K value (Finkentscher) of any particular mixture of polymers is calculated from viscosity data and is useful as an indication of the average molecular weight of such mixture. Its determination is'fully described in Modern Plastics, 23, No. 3, 157-61, 212, 214, 216, 218 (1945) and is defined as 1000 times k in the empirical relative viscosity equation:

log a rel 75 k C 1+1.5kC

wherein C is the concentration in grams per hundred cc. of polymer solution and 1 rel is the ratio of the viscosity of the solution to that of pure solvent. The K values are reported as 1000 times the calculated viscosity coefiicient in order to avoid the use of decimals. For the purpose of the present invention, there may be employed those polymeric N-vinyl lactams having a K value of about to 200, and preferably of about 20 to 100.

K values and specific viscosities (1; sp.) are interconvertible and are related through relative viscosity (17 rel.). Thus, when viscosity measurements are taken on solutions which have a concentration of 1.00 gram of polymer per deciliter of solution at 25 C. (C=1), the relationships are as follows:

1; rel.=n sp. plus one Relative viscosity=specific viscosity plus one Hence 1+1()[0.001K+0.000075K /(1+0.0015K)] Relative i it 10[0.001K+0.000075K 2 /(1+0.0015K)] Relative viscosity, specific viscosity, and K are dimensionless, whereas inherent viscosity (log n rel) and intrinsic viscosity (the limit of inherent viscosity as C approaches zero) have the dimensions of dilution, i.e., the reciprocal of concentration. Intrinsic viscosity and K are intended to be independent of concentration.

The number of recurring polymer units enclosed by brackets in the foregoing general structural formula, indicated by n, or the extent or degree of polymerization, corresponds to a chain of roughly 4 to 20,000 monomer units or more. In actual practice, a mixture of polymeric molecules, each containing a different number (n) of monomer units, is always produced. These homopolymers are readily prepared by the procedural steps given in U.S. Patents 2,265,450, 2,317,804 and 2,335,454 in which working examples of species included Within the above formula are given and which are incorporated herein by reference thereto.

Water-soluble copolymers operative in the instant invention are derived from polymerizable monomeric compositions containing at least about 20% by weight of an N-vinyl lactam. The proportion of N-vinyl lactam present in such polymerizable monomeric compositions will depend upon the amount and nature of the other monomer or monomers, but should in all cases be at least sufficient to impart to the final copolymer the desired water-soluble property. By way of example, the following list gives the maximum limit of'representative monomers which may be copolymerized with the N-vinyl lactam for the production of operable copolymers:

Vinyl stearate 15 'The following table lists some monomeric compositions useful in the production of suitable water-soluble copolymers.

TABLE Percent (1) N-vinyl-Z-pyrrolidone 70 Allyl alcohol"- 30 (2) N-vinyl-Z-pyrrolidone 85 Diallylphthalate 15 (3) N-vinyl-Z-pyrrolidone 60 Vinyl ethyl ether 40 (4) N-vinyl-2-pyrrolidone 30 Vinyl methyl ether 70 (5) N-vinyl-Z-py-rrolidone 75 Vinyl acetate 25 (6) N-vinyl-Z-pyrrolidone 50 Acrylic acid 50 (7) N-vinyl-2-pyrrolidone 20 Maleic anhydri (8) N-vinyl-2-pyrrnlidnne 94 Vinyl stearate 6 (9) 3-methyl-N-vinyl-2-pyrrolidone 87 Vinyl bromide 13 (10) 3-methyl-N-vinyl-2-pyrrolidone 87 Vinyl chloride 13 (1'1) 4-methyl-N-vinyl-2-pyrrolidone 88 Vinyl chloride 12 (12) 3,3-dimethyl-N-vinyl-2-pyrrolidone 91 Vinyl chloride 9 It will be understood that the above-described copolymers are generally prepared in the same manner as the homo-polymers of the N-vinyl lactams and have similar properties with respect to water-solubility, K values and the like.

Instead of copolymers prepared from mixtures of monomers as described above, a water-soluble polymeric material may be employed which is produced by mixing the polymerized N-vinyl lactam with one or more of the polymerized monomeric unsaturated compounds employed in producing such copolymers provided of course that the mixture contains at least 20% of polymerized N-vinyl lactam, is water soluble, and has the above-described properties with respect to K values and the like.

The preferred N-vinyl lactams for use in the compositions of the instant invention are N-vinyl-2-pyrrolidone and its lower alkyl substituted derivatives, which may be characterized by the formula:

( JH=CHz in which one or more of the hydrogen atoms in the cyclic methylene groups may be substituted by a lower alkyl radical such as methyl, ethyl or the like.

The means which may be employed to etfect the insolubilization of the polyvinyl lactam-containing polymeric substances on the inorganic fiber or flake include the following chemical means:

(a) Polybasic acids: Tannic acid Maleic acid Malonic acid Adipic acid Succinic acid Phthalic acid Phenols: Resorcinol Alkylated phenols e.g., nonyl phenol (c) Persulfates:

Ammonium persulfate Sodium persulfate Potassium persulfate (e) Isocyanates:

E.g., tolidine diisocyanate 2,4-tolylene diisocyanate 1,5-naphthalene diisocyanate 4,4-diphenyl diisocyanate 1,4-xylylene diisocyanate The inorganic materials which are contemplated and operable in the present invention include both natural and synthetic inorganic substances such as glass fibers, silica fibers, ceramic fibers, asbestos fibers, and flake materials such as ceramic flakes, glass flakes, mica and the like and mixtures of these with cellulosic fibers and other synthetics normally employed in paper-making. In addition to mixtures of inorganic fibers and the like with cellulosic fibers, it is possible to employ mixtures containing, in addition to the cellulosic fibers or in lieu thereof, other natural and synthetic fibers and filaments, such as cellulose acetate, poly .acrylonitrile fibers, terephthalate polyester fibers, nylon fibers (that is, hexamethylene adipamides) and the like. The amount of polyvinyl lactam-containing polymeric material which may be employed in the process and products of this invention is not critical. As little as 0.1% based on the weight of the inorganic stock gives outstanding results. As much as or more may be used but the preferred range based on the weight of the inorganic stock is from about 0.2% to about 5%. The larger amounts may be used if desired but are not necessary in order to achieve the advantages of this invention. The amount of chemical insolubilizing agent used may vary but the stoichiometric quantity necessary to effect complete insolubilization of the amount of the polyvinyl lactam-containing polymeric material present is preferred although greater amounts may be used. of the various chemical insolubilizers, those which are preferred are the polymeric compounds containing carboxyl or anhydride groups, and more particularly, copolymers of vinyl alkyl others with maleic anhydride. Such copolymers are water-soluble materials and any of these may be used wherein the molecular weights are above about 300. Those which are preferred, however, have molecular weights within the range of about 1000 to 10,000. Such molecular weights are representative of those copolymers which have a specific viscosity of about 0.1 to about 2.0, and these in turn represent K values of about to 75. It is of course understood that copolymers of higher molecular weights may be used provided they satisfy the characteristic of water-solubility.- Thespecific viscosity of these copolymers is determined as described above for the N-vinyl lactams employing a solution of 1 g. of polymer per 100 ml. of'2-butanone at C.v The following examples will serve to illustrate this invention Without being deemed limitative thereof. In these examples parts are on a weight basis except as otherwise indicated. Example 1 1 g. of blown silica fibers is added to 100 ml. of water. The pH of this fiber-water mixture is substantially neutral. The silica fibers are characterized as consisting essentially of 100% silica and physically they average from about /2 in. to 1 in. in length and have an average diameter of approximately 0.5 micron. The mixture is then warmed until the temperature thereof is F. To this heated mixture there is then added, based on the weight of the silica fibers present, a quantity of polyvinyl pyrrolidone (K=20 equal to about 1%, that is, about 10 mg. of polyvinyl pyrrolidone are added. The mixture is then subjected to vigorous agitation for about 2 minutes, Whereupon the fibers are broken down to a length ranging from about in. to about A in. To this mixture there is then added as a 5% aqueous solution sufficient of copolymer of vinyl methyl ether and maleic anhydride (K=20') to chemically insolubilize all of the polyvinyl pyrrolidone present in the mixture. About 5 to 10 drops of the aforementioned aqueous solution is suflicient to accomplish this result. The pH of the mixture is then adjusted to 2.2 after which it is permitted to stand with mild agitation of the solids for several minutes to permit any entrapped air to dissipate from said mixture. The volume of the resultant slurry is then increased about tenfold with water at a temperature of about F. after which the slurry'is poured into a standard sheet mold and the .wet 'web couched from a 200 mesh screen. The resultant sheet contains about 0.5% polymer content and has a breaking strength of 10.2 lbs. per sq. in.

Example 2 Example 4 The procedure of Example 1 is repeated except that a is employed in lieu of the silica fibers of that glass fiber example. The physical and chemical characteristics of the glass fiber are as follows: Chemical composition: Percent SiO V 54 C80, 16 i A1203 F203 B 0 10 Na O and K 0 5 Physical characteristics:

Fiber length 52 to Fiber diameter 0.50 micron. Fiberappearance.. White, soft fibers,

The sheet resulting from this process is similar in its polymer content to that of Example 1. The breaking strength is found to be 27 lbs. per sq. in.

Example 5 Example 7 1 g. of a batch of glass fiber is added to 100 ml. of water. The glass fibers have the same chemical composition as that of Example 4. The physical characteristics, however, are as follows:

Fiber length V to /s in. Fiber diameter 1.25 micron. Fiber appearance White, coarse fibers.

The procedure of Example 1 is then followed to effect the formation of a finished sheet using a pH of 1.9. The breaking strength of this sheet is 23 lbs. per sq. in.

In the following examples the procedure of Example 1 is followed except that the polymers of vinyl pyrrolidone which are used differ in their K values and also in the chemical substituents present therein. In addition, various chemical insolubilizers are used in lieu of the copolymer of Example 1. These examples are tabulated in Table I below:

TABLE I N-vinyl lactazn Insolubilizer PVP 1 (K=25) PVM/MA 2 (-K=30). PVP (K=35) PVM/MA 2 (K=20).

PVIVI/MA 2 (K=55). PVlVI/MA Z (K=75). PVE/MA 3 (K=30). PVE/IVIA 3 (K=75). PVE/MA 3 (K=30). PVld/MA (K=30).

PVE/Ii/IA (K=75).

PVDMP 6 (K=95) 1 Polyvinyl pyrrolidone.

2 Copolymer of vinyl methyl ether with maleic anhydride.

3 Copolymer of vinyl ethyl ether with maleic anhydride.

4 Polyvinyl-3411ethyl-apyrrolidone.

b Copolyrner of vinyl-n-propyl ether with maleic anhydride. Polyvinyl-3,3-dimethyl-2-pyrrolidone.

1 Copolymer of vinyl isobutyl ether with maleic anhydride.

The breaking strength of the sheets prepared in Examples 8-20 are as follows:

Examples 21-33 The procedures of Examples 8 through 20 are repeated except that the pH is adjusted to 4.0. In each instance the breaking strength of the resultant sheets is about M of the corresponding products of Examples 8 through 20; in other words, the strength of the sheets of Examples 21 through 33 range from about 1.0 lb./in. to 1.2 lb./in.

Example 34 The procedure of Example 1 is again repeated except that there is used in lieu of polyvinyl pyrrolidone a copolymer of vinyl pyrrolidone (75%) and vinylacetate (25%) (K=52). The resultant sheet is comparable to that of Example 1.

Examples 35 through 46 In the following examples the procedures of Examples 1 and 4 are repeated, that is, using silica fibers on the one hand and glass fibers on the other hand in the procedures of these examples, employing, however, varying copolymers containing and N-vinyl lactam, and also where indicated, difierent maleic anhydride copolymers.

TABLE III Ex- Fiber N-vinyl lactam K Insolubilizer ample copolymer 35 silica VP/VM (30:70) 25 PVM/MA (I =30). 3 VP/VM (30:70) 25 PVM/MA (l=30). VP/VE (60:40) 3 1 PVM/MA (K=30). VP/VE (60:40) 34 PVM/MA (K=30). VP/AA (50:50) 40 EVE/MA (K=75).

A (50:50) 40 PVE/MA (K=75). VMP/VO (87:13) 42 PVE/MA (K=75). VMP/VO (87 13) 42 PVE/MA (K=75). VP/VS (94.6) 38 PVPn/MA (K=15). glass VP/VS (94:6) 38 PVPH/MA (K=45). 45 silica VP/VA (75:25) 30 PVPn/MA (K=45). 46 glass VP/VA (75:25) 30 PVPn/MA (K=45).

1 Copolymer of vinyl pyrrolidone with vinyl methyl ether; monomer contents as indicated.

2 Oopolymer of vinyl pyrrolidone and vinyl ethyl ether.

3 Copolymer of vinyl pyrrolidone and acrylic acid.

4 Gopolymer of vinyl-3-1nethyl pyrrolidone and vinyl chloride.

5 Copolyrner of vinyl pyrrolidone and vinyl stearate.

Copolymer of vinyl pyrrolidone and vinyl acetate.

ziOoprgymer of vinyl methyl ether and maleic anhydride; K value as in icate 8 Oopoly'mer of vinyl ethyl ether and maleic anhydride.

9 Copolymer of vinyl-n-propyl ether and maleic anhydride.

The results obtained from the above example are comparable in so far as the resultant sheets are concerned to those of Examples 1 and 4.

Examples 47 and 48 The procedures of Examples 1 and 4 are repeated except that the pH employed is 2.5 instead of 2.2. The final products have breaking strength of 10.4 and 26.5 1b./sq. in., respectively.

Examples 49 and 50 The procedures of Examples 1 and 4 are again repeated except that the temperature is 650 F. instead of 850 F. The sheets produced in accordance with such processes are substantially identical in their characteristics to those obtained in their characteristics to those obtained in Examples 1 and 4.

Example 51 The procedure of Example 1 is repeated using asbestos fibers in place of the silica fibers of that example. The asbestos fibers are blue, soft fibers characterized by hav ing fiber lengths varying from about /s to A2 in. and fiber diameters averaging about 0.05. The breaking strength of these sheets is 5 3 lb./ sq. in.

Example 52 The procedure of Example 2 is repeated employing the same asbestos fibers as in Example 51. The sheets resulting from this process have a breaking strength of about 1.5 lb./ sq. in.

Examples 53 and 54 The procedures of Examples 1 and 4 are repeated except that 10 drops of a 10% aqueous solution of maleic acid is used as the chemical insolubilizer. The results are comparable to those OfExamples 1 and 4.

Examples 55 and 56 Examples 1 and 4 are again repeated using in lieu of the chemical insolubilizer of those examples in each instance, 20 drops of a 5% aqueous resorcinol solution. Comparable sheets result as in Examples 1 and 4.

Examples 57 and 58 The procedures of Examples 1 and 4 are further repeated using 12 drops of a 10% aqueous ammonium persulfate solution instead of the insolubilizer of those examples. The resultant sheets exhibit similar characteristics as those of Examples 1 and 4.

Examples 59 through 62 The procedures of Examples 1 and 4 are repeated using as the chemical insolubilizer (a) 1 ml. of a 5% aqueous solution of a styrene maleic anhydride and (b) /5 m1. of

a solution of 2,4-tolylene diisocyanate in chloro benzene. Results similar to those of Examples 1 and 4 are obtained.

Example 63 tion of polyvinyl pyrrolidone (K=20) is added. The mixture is then further agitated for l min. To half of this mixture there is added 0.5 ml. of a 5% aqueous solution of a copolymer of vinyl methyl ether with maleic anhy- 'dfid'e (K=20), after which the pH is lowered to 2.2. Further processing is carried out as described in Example 1 to form a sheet. This sheet resembles newsprint in hand strength and flexibility. The remaining half of the mixture is used in a similar manner except that maleic acid is used as the insolubilizer. A similar sheet results.

Example 64 2 g. of mica fiakes of miscellaneous dimensional characteristics and averaging about 2 mo in thickness and whose chemical composition is are dispersed with vigarous agitation in 250 mls. of water (pH=6.5 The temperature of the mixture is then raised to 90 F. and there is then slowly added while agitation is continued 1.5 ml. of a 10% aqueous solution of polyvinyl pyrrolidone (K=60. Agitation is continued for several minutes after all of the polyvinyl, pyrrolidone has been added, and then there is slowly added 2 ml. of a 10% aqueous solution of a copolymer of vinyl n-butyl ether with maleic anhydride (14:37). The pH is then lowered to 2.2 by the addition of hydrochloric acid and a sheet is then formed from this mixture in the manner described in Example 1. The resultant product shows exceptionally and unexpectedly high breaking strength of 12.4 lbs/sq. in.

Example 65 Repeating the procedure of Example 64 except that the pH is only lowered to 4.5 results in a sheet which has a breaking strength of only 1.5 lbs./sq.in.

Example 66 The procedure of Example 64 isagain repeated using glass flakes in place of the mica flakes of that example. Such flakes have a chemical composition similar to the glass fibers of Example 4. They have an average thickness of 15 [L and range in area from A; to /2 in. The resultant sheet has a breaking strength of 3.7 lb./sq.in. A similar sheet prepared at a pH of 4.2 has a breaking strength of less than /2 lb./sq.in.

In addition to the materials, compounds and compositions which have been specifically exemplified above and further, in addition to those processing conditions which have been specifically shown in these examples, it is clear and within the purview of this invention to employ all of the other materials, compounds and compositions here disclosed and their equivalences, and it is also possible to employ other conditions than those specifically shown but which come within the scope of the conditions taught herein. Further, while this invention has been disclosed and exemplified with respect to siliceous fibers and flakes, other inorganic fibers of nat- 10 ural or synthetic origin and other flake materials equally operable.

Variations and modifications which 'will be obvious and apparent to those skilled in the art may be made in the procedure above described without departing from the scope and spirit of my invention. I I

I claim: 7 1. In a process for preparing a sheet-like product from siliceous stock material comprising incorporating into said stock from about 0.1 to about 10% of a chemically will be insolubilized N-vinyl lactam containing polymeric substance and then forming a sheet from said stock, the improvement which comprises maintaining the pH of the said stock material at from about 1.5 to about 2.5 during the incorporation of the polymeric substance.

2. In a process for preparing a sheet-like product from siliceous stock material comprising incorporating .into said stock from about 0.2 to-about 5% of a chemical- 1y insolubilized N-vinyl lactam containing polymeric substance and then forming a sheet from said stock, the improvement which comprises maintaining the pH of the said stock material at from about 1.5 to about 2.5 during the incorporation of the polymeric substance.

3. In a process for preparing a sheet-like product from siliceous stock material comprising incorporation into said stock a chemically insolubilized polymeric substance containing at least 20% of an N-vinyl lactam .and then forming a sheet from said stock, the improvement which comprises maintaining the pH of the said stock matenial at from about 1.5 to about 2.5 during the incorporation of the polymeric substance.

4. In a process for preparing a sheet-like product from siliceous stock material comprising the steps of adding to said stock from about 0.1 to about 10% based on the weight of said stock of a water-soluble N-vinyl lactam containing polymeric substance, chemically insolubilizing said polymeric substance and then forming a sheet therefrom, the improvement which comprises adjusting the pH of the stock to from about 1.5 to about 2.5 prior to sheet formation.

5. A process as defined in claim 4 wherein the stock comprises silica fibers.

6. A process as defined in claim 4 wherein the stock comprises glass fibers.

7. A process as defined in claim 4 wherein the stock comprises mica.

8. A process as defined in claim 4 wherein the stock comprises asbestos fibers.

9. A process for preparing a sheet-like product from siliceous stock material which comprises adding to said stock from about 0.1 to about 10% based on the weight of said stock of a water-soluble homopolymeric N-vinyl lactam containing polymeric substance, chemically insolubilizing said polymeric substance, adjusting the pH of the stock to from about 1.5 to about 2.5 and thereafter forming a sheet from said stock.

10. A process as defined in claim 9 wherein the chemical insolubilization is efiected by means of a carboxyl containing polymeric substance.

11. A process as defined in claim 9 wherein the chemical insolubilizer is a copolymer of a vinyl lower alkyl ether with maleic anhydride.

12. A process for preparing a sheet-like product from siliceous stock material which comprises adding to said stock from about 0.1 to about 10% based on the weight of said stock of a water-soluble copolymer containing at least 20% of an N-v-inyl lactam, chemically insolubilizing said polymeric substance, adjusting the pH of the stock to from about 1.5 to about 2.5 and thereafter forming a sheet from said stock.

13. A process as defined in claim 12 wherein the chemical insolubi lization is eifected by means of a carboxyl containing polymeric substance.

14. A process as defined in claim 12 wherein the chem- 11 ical insolubilizer is a copolymer of a vinyl lower alkyl ether with maleic anhydride. 7

15. In a process for preparing a sheet-like product 'from siliceous stock material comprising the addition to an aqueous slurry of said stock of from about 0.1 to about 19% based on the weight of the siliceous stock of a water soluble polyvinyl pyrrolidoneflhe insolubilization of said polyvinyl pyrrolidone by the addition of a copolymer of a vinyl lower alkyl ether with maleic anhydride and then the formation of a sheet from said aqueous slurry, the improvement which comprises adjusting the pH of the aqueous slurry to from about 1.5 to about 2.5 prior to sheet formation.

16. In a process for preparing a sheet-like product from glass fibers comprising the steps of suspending said fibers in an aqueous medium, adding to said suspension 0.5% based on the weight of the glass fibers of a watersoluble polyvinyl pyrrolidone, thereafter adding to said suspension a sufiicient amount of a water-soluble copoly- 12 mer of vinyl methyl ether with maleic anhydride to insolubilize the polyvinyl pyrrolidone, and then forming a sheet from said suspension mixture, the improvement which comprises adjusting the pH of said suspension mixture to about 2.2 prior to sheet formation.

References Cited in the file of this patent UNITED STATES PATENTS 2,335,454 Schuster et al. Nov. 30, 1943 2,698,558 Hawley et al. Jan. 4, 1955 2,787,542 Labino Apr. 2, 1957 2,802,734 Bandel Aug. 13, 1957 2,901,390 Conklin et al. Aug. 25, 1959 OTHER REFERENCES Maxwell: Technical Association of the Pub. and

Paper Industry, May 13, 1943, pp. 39 to 41.

OLeary et al.: TAPPI, vol. 37, No. 10, Oct., 1954, pp. 446-450. 

1. IN A PROCESS FOR PREPARING A SHEET-LIKE PRODUCT FROM SILICEOUS STOCK MATERIAL COMPRISING INCORPORATING INTO SAID STOCK FROM ABOUT 0.1 TO ABOUT 10% OF A CHEMICALLY INSOLUBILIZED N-VINYL LACTAM CONTAINING POLYMERIC SUBSTANCE AND THEN FORMING A SHEET FROM SAID STOCK, THE IMPROVEMENT WHICH COMPRISES MAINTAINING THE PH OF THE SAID STOCK MATERIAL AT FROM ABOUT 1.5 TO ABOUT 2.5 DURING THE INCORPORATION OF THE POLYMERIC SUBSTANCE. 